Hadley's quadrant

octant

John Hadley invented his "reflecting quadrant" in 1731, the direct ancestor to the modern sextant. Called a reflecting quadrant for most of the 18th century and quadrant in 19th century America, it actually has an arc of 45 degrees hence its modern name of octant. Instead of working with sun shadows, the octant used mirrors to line up a reflection of the sun or other celestial object with the horizon. It could be used for star sights, which was a major advantage. It evolved into the sextant about 1757 but continued to be made into the middle of the 19th century.